See the images above. Look at the washed out blacks and ghosting on the Kobo and look at the deep blacks on the Kindle and Sony.

I played with cover images from several books in Photoshop and converted them to 16 gray level from colour. The Kobo comes closer to the results I got from the cover images I played with and converted with Photoshop's grey scale conversion. The images from the other reader looks like an image converted using a desaturation technique. I do remember printing black and white photographs on high contrast paper just because the images looked snappier but at the price of losing detail in the highlights and shadows.

There is an interesting tutorial on using GIMP to convert images to B&W which shows some of the differences using different methods of mixing the RGB components in the conversion process. There is also a 16 gray plugin for GIMP which I am going to have to try.